Conventional screw-type heat-exchanger systems typically include multiple rotary screw conveyers arranged in parallel (with co-planar axes) with overlapping flights. These heat-exchanger systems are commonly used for heating (or cooling) and conveying (and sometimes mixing) non-flowable solid materials and flowable materials such as a slurry (a thick mixture of a solid suspended in a liquid), another heterogeneous material, or a bulk solid/commodity. In such heat-exchanger systems, the heat-exchange surfaces of the helical flights and the screw shafts contact and exchange heat with the material being processed.
Because each screw conveyor in the system has helical flights that overlap with the helical flights of an adjacent screw, the screw conveyors must be operated in a coordinated manner. That is, the rotational position of each screw (and thus the axial position of its flighting) must coordinated relative to the rotational position of each adjacent overlapping screw (and thus the axial position of its flighting), a feature known as clocking. If one screw were operated independently of the other, without clocking, its flighting would eventually interfere/collide with the flighting of the adjacent screw. In large heat-exchanger systems, such a collision can result in a catastrophic failure.
To provide this clocking feature, traditional multi-screw heat-exchanger systems include bull gears, with the flights of adjacent screws in a fixed axial relationship (fixed clocking). A drive system rotationally drives one of the screws directly, and the bull gears transmit the power from the directly driven screw to the other/non-directly driven screw (or screws) so that both screws rotate at the same constant speed.
FIGS. 1-3 show one such prior-art multi-screw heat-exchanger system 10 with fixed clocking. The system 10 includes two screw conveyors (a directly driven screw 20a and a non-directly driven screw 20b, collectively “the screw conveyors 20”), a screw-mounting assembly 30 for each screw, a screw-drive assembly 40 for driving the screws, and a control system 50 for operating the screw-drive assembly. The screw conveyors 20 each include a rotary shaft 22 with a helical flight 24 extending radially outward from the shaft so that the flights overlap with each other. The screw-mounting assemblies 30 include, at the head end of each screw 20, a rotary drive shaft 32 extending axially from the respective screw shaft 22 and two rotary bearing pairs 34 mounted on that drive shaft. The screw-mounting assembly 30 also includes, at the tail end of each screw 20, a rotary drive shaft (not shown) extending axially from the respective screw shaft 22 and one rotary bearing pair (not shown) mounted on that drive shaft. The screw-drive assembly 40 includes, for driving the directly driven screw 20a, a gear-motor 42, drive and driven sprockets 44 operably coupling the gear-motor to the drive shaft 32, and a chain 43 operably coupling the sprockets together. The screw-drive assembly 40 also includes, for driving the non-directly driven screw 20b, a bull gear 45 that is mounted on and rotational with each drive shaft 32 and that is positioned between the respective bearing pairs 34, with the bull gear on the directly driven screw 20a in meshing engagement and driving the bull gear on the non-directly driven screw 20b. And the control system 50 includes conventional controls for operating the gear-motor 42 to drive the screws 20 at a constant speed.
Additional details of screw-type heat-exchanger systems are disclosed in U.S. Pat. No. 5,417,492, issued May 23, 1995; U.S. Patent Application Pub. No. US2008/0295356, published Dec. 4, 2008 (U.S. Non-Provisional patent application Ser. No. 12/156,681, filed Jun. 2, 2008); and U.S. Patent Application Pub. No. US2010/0051233, published Mar. 4, 2010 (U.S. Non-Provisional patent application Ser. No. 12/552,369, filed Sep. 2, 2009), all of which are hereby incorporated by reference herein.
The bull gears traditionally used for timing/clocking of the screws each have two bearings on the screw shaft, one on each side of them, and these bearings must be assembled in precise alignment with a tail bearing on the other (non-directly driven) end of the screw. When the heat-exchanger system is assembled, the relative position of the screws (i.e., the clocking) is set by precisely keying the bull gears to the screw shafts. Any subsequent maintenance adjustment requires dismounting at least one of the gears, rotating the respective screw to the desired position, and remounting the respective bull gear. Since the driving force for the non-directly driven screw is transmitted through the bull gears, and the screws are typically very large and heavy, the bull gears are necessarily very large and heavy. Precise field re-alignment of such large and heavy gears and bearings can be extremely difficult and time-consuming.
In addition, because each screw has a bull gear, and because adjacent meshing bull gears rotate in opposite angular directions, the screws of two-screw systems counter-rotate with respect to each other (that is, they rotate in opposite angular directions). In systems with more than two screws, adjacent screw pairs (with one driving the other) rotate in opposite angular directions. This counter-rotation of adjacent screws produces relatively little mixing action of the material being processed.
Furthermore, multiple overlapping screw conveyors are typically used in these applications because they can usually move the material satisfactorily, even if the screw conveyers are significantly inclined upward from input (head) to output (tail) ends, as long as the material is not extremely flowable. However, if the material is a heterogeneous mixture of, for example, fibrous materials and fine powders, the fibrous material can segregate and accumulate in the screw conveyers, while the more conveyable granular material is rapidly discharged from them. In addition, for maximum heat-exchange capacity and efficiency, the heat-exchange surfaces must be directly and completely (or substantially so) covered with the material. But if the material is sticky, doughy, or otherwise non-flowable, or undergoes a phase change in which it becomes so, the material can build up and bake onto the heat-transfer surfaces of the flighting such that it significantly reduces heat-transfer efficiency. And such sticky and plastic material can fill the inter-flight voids defined by adjacent screws and form a log of material that rotates with the flights with very little axial motion. Furthermore, very free-flowing materials can flow backwards if installed on upwardly inclined screw conveyors, or such materials can outrun the flighting of downwardly inclined screw conveyors and flow out the outlet end thereby leaving the top part of the screws unused as heat exchange area. Moreover, some materials (e.g., biomass cells) tend to form into chunks or clods with wet centers and dried crusts that prevent the drying of the encrusted wet material, and this problem is traditionally managed by recycling fifty percent of the cloddy material back through the equipment, thereby increasing the size, complexity, and energy requirements of the screw-conveyer system.
One known prior system that attempts to overcome some of these problems includes a set of eccentric bull gears that cause the screw flights to counter-rotate (only) and to advance and retard axially relative to each other during each screw revolution in an attempt to clean the material accumulation from the flights for improved heat-transfer efficiency. This system provides a constantly varying clocking during each revolution, but cannot adjust the clocking to a specific fixed position or adjust the clocking over more than one revolution, and the maintenance problems relating to bull gears remains.
Another such known prior system includes chains and sprockets (with idler sprockets for tensioning and positioning to produce adequate “rap” on the primary sprockets) for co-rotating the screws in the same direction and providing the needed clocking. This co-rotating screw arrangement tends to provide for increased mixing action of the material being processed. With co-rotation, the mixing is enhanced because the travel length of the material in the flight overlap area is increased due to the overlapped flight sections approaching each other from, and withdrawing from each other in, opposite angular directions. In addition, this design does not include the bull gears or their bearings for clocking. However, these systems provide only fixed clocking and they suffer from the same maintenance problems as with systems with bull gears (because they have the same three-bearing design for the sprockets and also require additional bearings for the idlers.
Accordingly, it can be seen that needs exist for improvements to multi-screw heat-exchanger systems and/or drive control systems for them. It is to the provision of solutions to this and other problems that the present invention is primarily directed.